Overload relay



J. H. ST. JOHN OVERLOAD RELAY Feb. 10, 1948.

Filed July 7, 1942 INVENTOR. John H St Jb/un BY Y n 77'0/2 NE Y Patented F b. 1o, 1948 UNITED- STATES PATENT OFFICE OVERLOAD RELAY John H. St. John, Rutherford, N. J., assignor to Beyer Industries Incorporated, Belleville, N. J a corporation of Delaware Application July 7, 1942, Serial No. 449,982

6 Claims. (01. 175-336) This invention relates to a new and useful improvement in overload relays for automatically making or breaking an electrical circuit when the current in it rises to a predetermined value.

In many applications of electricity it is essential to protect a circuit or apparatus from the effects of current fluctuations, particularly current increases or overloads. Many operations electrically performed can tolerate an overload of only a very few percent. Therefore, such overload relays must operate positively within a relatively small margin of error if they are to supply the essential protection.

The common type of overload relay depends for its action upon the attraction of -an armature by an electro-magnet against the substantially constant mechanical resistance of a spring. The breaking point or current value at which. the relay operates is determined by the strength of the spring. If the current tends to rise very sharply to far beyond the breaking point, this 1: vpe of relay will operate more or less satisfactorily to definitely break the current at some pre-set value. When the current is A. C. or pulsating D. C. and hovers within a close range of the overload setting, a condition is reached when the attraction of the magnet is of too short duration to fully overcome the inertia of the armature and the resistance of the spring, with the result that the armature tends to vibrate or chatter so that the circuit is not postively broken, but is alternately broken and closed in rapid sequence. This causes wear on the relay, which is rapid and serious, and also sets up an objectionable buzzing or chattering sound. Various devices, such as dash-pots, have been applied to such relays to overcome this difliculty.

This invention relates to another and superior method of preventing this chattering. In the circuit-breaker of this invention, although a spring is employed to hold the armature away from the electro-magnet when the relay is inert, and also for making adjustments, it is not the sole holding means as the operation of the relay is controlled primarily by opposed magnetic forces, one of which opposes motion by the armature, and the other of which attracts it. As the .current increases, both of these forces gain in strength, but the attractive force increases more rapidly than the opposing force, the relative rates of increase being such that when the breaking point is reached, the attractive force is greater than the opposing force, or, more accurately, since the resistance of the spring has some effect, the attracting force then exceeds the combined resistance of the opposing force and the spring. The holding force of the spring points.

is constant with increased current,.but the holding force of the magnetic back-stop, which is the important feature of this invention, becomes greatly weakened as the armature starts away from it, and the armature, therefore, snaps quickly to the opening position of the contact Because of this non-chattering feature, the overload relay of this invention can be relied upon to break the circuit within a few percent of the predetermined current, value, which is as close as any commercial instrument can be expected to operate. Also, it is extremely simple and inexpensive to manufacture, and. because of its substantially chatterless characteristic, is very durable.

One form of the invention is shownin the accompanying drawings, of which:

Figure 1 is a front elevation of the overload relay: and

Figure 2 is a plan view of the relay.

The overload relay shown in Figure 1 consists of an L-shaped frame I on the end of the horizontal leg of which is mounted the core of an electro-magnet 3. This core is in circuit with the frame and may be attached thereto by a simple bolt 4. The core consists of a main body 5, which, for convenience, will be called hereafter the attracting" core. Around this core and insulated therefrom the winding 6 is positioned. I

The number of turns in this winding will, of course, be determined by the strength of the current employed and the intensity of the magnetic field desired in accordanc with established electrical practice. Above attracting core 5 and integral therewith is an extension I, terminating in a head 8, which will be hereinafter called the resisting core.

The armature 9 is pivotally mounted on the end of the other leg ill of the frame. The pivotal mounting may be of the simple and very inexpensive type shown, produced by forming a shallow fiat recess II in the end of leg Ill, and notching the edges of armature 9, as at i2, So that the armature will fit into the recess and the notches will be loosely engaged by the sides thereof.

To one end I3 of armature 9 is attached a spring M, which may also be attached to a suitable pin l5 ,projecting from the frame. The major purpo e of this spring is to urge the other end of the armature upward when the relay is inert. Since the strength of this spring may not always conform exactly to specification, an adjustment may be provided for it if desired, such as the adjusting screw i8 and lock nut i! by which the tension oi. spring II can be regulated.

The other end of armature 8 is forked, as at I8, and core extension 1 is positioned within this fork. Resisting core 8 at the upper end of extension 1 is larger than fork [8 so that it acts as a stop to limit the upward motion of armature 8 at all times. In other words, armature 9 can rock within the space formed between the top or attractin'g core 8, and the bottom of resisting core 8, and will normally rest against the bottom of the latter, and will, consequently, be separated from attracting core 8 by an air space.

Although this type of pivotal mounting substantially restricts the motion of the armature to a vertical plane, the top of attracting core is also provided with a non-magnetic pin I9,

which makes a sliding fit within the fork I8 so that all lateral motion by the armature is prevented and its alinement with the magnet core assured.

Upon armature 8 and insulated therefrom are spring contacts 20 and 2|, which are normally closed when the relay is otherwise inert. These contacts are connected into the circuit which the relay is required to break, as indicated by wires 22 and 23. Contact 2| is provided adjacent its free end with an insulating button 28 adapted to rest upon the top of resisting core 8 or the ma net. This button prevents contact 2i from moving downwardly without interfering with its upward motion so that positive engagement between the two contacts is assured. when the electromagnet is charged suiliciently to cause the armature to move downward, obviously contact 20 will move with it, and since contact 2| cannot move in this direction beyond the limit set by button 24, it will separate from it and the circuit 22, 23, will be broken. Other suitable contact devices may, of course, be substituted.

The operating accuracy of this relay which, when used with a pulsating D. 0., or an alternating current may be within two percent, results primarily from the peculiar nature of the cores of the magnet and the relation of the armature thereto. As previously stated, the core of the magnet consists of a main body of two major parts, an attracting core 8 and a smaller resisting core 8 connected together by an integral extension 1. Between cores 5 and 8 the free end of armature 9 is rockably positioned, and is in contact with the underside of core 8 when the relay is not operating.

When current passes through the magnet winding 8 the armature 9 is acted upon by two magnetic forces in addition to the incidental mechanical force of the spring l8. Resisting core 8 being a part of the magnet core, and, consequently, of the magnetic circuit, exerts a holding force on the armature which is in contact with it, commensurate with the area of contact and the flux density in the iron. Simultaneously, the attracting core underneath the armature is exerting a force of a value depending upon its area, the width of the air gap between it and the armature and the flux density. The armature will not move until the holding power upon armature 9 of the resisting core 8 and the minor resistance of spring H are overcome by the attractive force of the attracting core 8. Since the attracting area of the attracting core 8 is larger than that of resisting core 8, an increase in the current in winding 8 will cause a greater increase in the attractive power of core 5 than of core 8. Consequently, when the current has become large enough, the attraction or core 8 will overcome the holding power 01' core 8 and the resistance of spring I8, and the armature will'be pulled away from core 8 towards core 8.

As soon as armature 8 has separated from core 8 by an even minute distance, the holding power 01 core 8 decreases instantaneously to only a small fraction or what it was when the armature was in contact with it. Therefore, since the attractive power of core 8, which already exceeded the holding power or core 8 and the resistance of spring I 4 sui'nciently to start the movement of the armature, now becomes very great, relatively speaking, and the armature is pulled with great positiveness down against the core 8. As a matter-oi-i'act, as the armature moves, thereby reducing the air gap between it and core 8, and increasing the air gap between it and core 8, the attractive power of core 8 increases very rapidly, whereas the resisting power or core 8 decreases very rapidly so that the relative eilects of the two cores acting upon the armature are increasingly disproportionate as the operation progresses.

Since chattering cannot occur until the armature has moved to some extent, it will be evident that in the relay 0! this invention chattering is practically impossible. The resisting corel tends to prevent chattering or armature 8 until the electro-magnetic force oi attracting core 8 starts it moving. Then the elect is not to chatter. but to immediately move completely against attracting core 8 for the reasons iust stated. It will also now be evident that the current value at which the relay operates is determined primarily by the relative proportions o! the attractive areas of cores 8 and 8, the width or the air gap between armature 8 and the core 8. the area of contact between armature 8 and core 8, and to a less d gree the resistance of spring l8. By properly proportioning these elements, the relay ma be made to operate substantially at any desired current value. Proportions both actual and relative may be computed from data generally available to electrical engineers, or empirically.

This type or relay may be made to operate with an error of about 1' two percent at the current value for which it is designed. Its life is indefinite, since wear due to chattering is negligible. Although small in size, its construction is rugged, and, with the exception of the core, requires no precision operations in its manufacture.

This relay may, obviously, be utilized as a circuit maker instead of circuit breaker by a simple modification of contacts 2| and 22, so that the circuit controlled by the relay will be made instead of broken when the current causes the relay to operate as described above. Also, a plurality of contacts may be used.

I claim:

1. In an overload relay, an electro-magnet having a core provided with a pole lace at one end thereof, a single winding for magnetizing said core, said core having an axial extension of a cross-sectional area considerably less than that of the core projecting from the pole face, a head on the free end of said extension of a crosssectional area slightly larger than that of the extension, and an armature having a slotted portion embracing the extension and movable between the pole face and said head.

2. In an overload relay, an electro-magnet having a core provided with a pole face at one end thereof, a single winding for magnetizing said core, said core having an axial extension of a cross-sectional area considerably less than that of the, core projecting from the pole face, a head'on/the free end of said extension of a cross-sectional area slightly larger than that of the extension, and an armature having a slotted portion embracing but out of contact with said extension and movable between the pole face and said head, and non magnetic means in said slot for guiding the armature in its movement be-' tween said pole face and head.

3. In an overload relay, an electro-magnet having a core provided with a pole face at one end thereof, a single winding for magnetizing said core, said core having an axial extension of a crosssectional area considerably less than that of the core projecting from the pole face, a head on the free end of said extension of a cross-sectional area slightly larger than that of the extension, an armature having a slotted portion embracing the extension and movable between the pole face and said head, and adjustable means urging the armature towards the head.

4. An overload relay comprising an electromagnet having a core provided with a pole face at one end thereof, a head in magnetic circuit with the core and spaced from the pole face, a single winding adapted to magnetize the pole face and the head with more lines of flux in the pole face than the head, an armature adapted to move between the pole face and the head, a non-magnetic member carried by the pole face and having a sliding flt against said armature to hold said armature in alignment, and means urging the armature toward the head whereby under normal conditions the additive forces of said means and the magnetic flux in the head will hold the armature against the head and under abnormal conditions the magnetic flux in the pole face will move the armature away from the head.

5. An overload relay comprising an electromagnet having an attracting core, a resisting core of lesser magnetic strength than the attracting core, a winding for energizing both cores, an armature adapted to move between the attracting and resisting cores, 9. non-magnetic member carried by the attracting core acting as a guide to prevent lateral movement of the armature to keep it in alignment between the attracting and resisting cores, and a spring urging the armature toward the resisting core and holding it close thereto whereby under normal conditions the additive forces of said spring and resisting core will hold the armature adjacent the resisting core and under abnormal conditions the attracting core will move the armature toward it against the tensioning of the spring.

6. An overload relay comprising an electromagnet having an attracting core, a movable armature having an aperture therein, a resisting core of lesser magnetic strength than the attracting core and against which the armature is adapted to lie closely adjacent under normal conditions, a support member for supporting the resisting core mounted on the attracting core and lying within the aperture in the armature, a spring adapted to urge the movement of the armature toward the resisting core, means to adjust the tension of the spring, and winding means for energizing both cores whereby when the attracting core has overcome the combined forces of the spring and the resisting core the armature is positively moved without chattering toward the attracting core.

JOHN H. ST. JOHN.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number 

